The invention relates to a zoom, or variable magnification lens and, more particularly, to a finite conjugate 10-element symmetrical zoom lens covering a half-field angle over 39.degree. and particularly suited for operation in a photocopying environment.
Photographic zoom lenses are well known in the art, the great majority operating over a half-field angle of less than 25.degree.. These lenses are characterized by being asymmetric and having conjugates to infinity. By contrast, zoom lenses used in photocopying devices tend to group the lens element comprising the lens symmetrically about a center stop position to reduce the effects of coma, lateral color or distortion. These lenses also operate with a finite conjugate. In prior art devices zoom lenses typically comprise multiple elements arranged symmetrically about a central aperture with one pair of symmetrically oriented lenses being movable.
The zoom lens must be designed to maintain total conjugate and focus as it goes through the zoom magnification range. A finite conjugate lens, as used in photocopier applications, thus normally requires an overall lens motion and some motion of lens elements relative to each other. U.S. Pat. Nos. 4,061,419, 3,728,010 and 4,406,522 are representative of lenses which undergo two motions, a lens grouping adjustment and the lens motion.
A remaining problem in prior art lenses is the requirement to compensate for various aberrations. Spherical aberration, axial color, astigmatism and field curvature add, rather than cancel between the front and back halves of a symmetric lens. Some parts of astigmatism can cancel between the two halves, but some cannot. Therefore, this second group of aberrations needs to be more highly corrected in each half. As a symmetric lens zooms to a magnification different from 1.times., it is no longer working at symmetric conjugates. This causes an imbalance in coma, lateral color, and distortion between each half of the lens which results in non-zero values of these aberrations. These problems are particularly acute in finite conjugate lenses operating over wide field angles.
The zoom lens of the present invention is designed to compensate for these aberrations, and particularly to compensate for the astigmatism variation as the lens zooms. This compensation is achieved by introducing an additional lens movement within the lens to vary the central aperture air spacing. While prior art devices are known which introduce a second lens element movement within the lens (e.g. U.S. Pat. Nos. 3,912,374, and 3,865,470); none accomplish the prescribed motion of the lens halves of the present lens design in order to correct the astigmatism and field curvature over a large field angle.